radar

Radar is a device to send and receive radio signals that bounces them off targets to obtain information about the targets. Radar is used on planetary missions and has also been used by radar located on or orbiting Earth that can reach the Moon and inner solar system planets.

The most basic information provided by radar is the distance to the target, derived from the time interval from sending to receiving a change in the signal, and the direction of the target using a highly-directional transmitter and/or receiver, e.g., with a dish. Circular polarization radar offers additional information about characteristics of the target.

Radio with a frequency of tens of megahertz (10 MHz-2.6 GHz) is not fully stopped by the ground, and radar using its penetration (called ground-penetrating radar or GPR) can offer information about the structure of moons and planets. Varying materials, e.g., underground layers, vary in their transparency to radio and create partial reflections, analogous to optical effects, information useful to determining and/or verifying what is below the surface.